Neuromolecular responses in disrupted mutualistic cleaning interactions under future environmental conditions

ABSTRACT Mutualistic interactions, which constitute some of the most advantageous interactions among fish species, are highly vulnerable to environmental changes. A key mutualistic interaction is the cleaning service rendered by the cleaner wrasse, Labroides dimidiatus , which involves intricate processes of social behaviour to remove ectoparasites from client fish and can be altered in near-future environmental conditions. Here, we evaluated the neuromolecular mechanisms behind the behavioural disruption of cleaning interactions in response to future environments. We subjected cleaner wrasses and surgeonfish ( Acanthurus leucosternon , serving as clients) to elevated temperature (Warming, 32°C), increased levels of CO 2 (High CO 2 , 1000 ppm), and a combined condition of elevated CO 2 and temperature (Warming & High CO 2 , 32°C & 1000 ppm) for 28 days. Each of these conditions resulted in behavioural disruptions concerning the motivation to interact and the quality of interaction (High CO 2 -80.7%, Warming – 92.6%, Warming & High CO 2 -79.5%, p <0.001). Using transcriptomic of the fore-, mid-, and hindbrain, we discovered that most transcriptional reprogramming in both species under warming conditions occurred primarily in the hind- and forebrain. The associated functions under warming were linked to stress, heat shock proteins, hypoxia, and behaviour. In contrast, elevated CO 2 exposure affected a range of functions associated with GABA, behaviour, visual perception, and circadian rhythm. Interestingly, in the combined Warming & High CO 2 condition, we did not observe any expression changes of behaviour. However, we did find signs of endoplasmic reticulum stress and apoptosis, suggesting not only an additive effect of the environmental conditions but also a trade-off between physiological performance and behaviour in the cleaner wrasse. We suggest that impending environmental shifts can affect the behaviour and molecular processes that sustain mutualistic interactions between L. dimidiatus and its clients, which could have a cascading effect on their adaptation potential and possibly cause large-scale impacts on coral reef ecosystems.